Wireless broadband networks make high performance Internet access possible where wired broadband infrastructure is impractical. However, such a wireless broadband network will only be a success especially for residential and small business markets if the infrastructure is provided at a low cost, is robust to changing environments, easy to deploy and scalable with market demand.
New wireless networks with wireless routers as network nodes on a mesh network basis emulate the topology and protocols of the Internet but are optimized for wireless high-speed data transmission. Such mesh network consists of wireless routers connected to each other with radio links. The radio links are able to arise and break down and the network is able to adjust itself to the current condition of the radio links. Also individual devices can be switched on and off without disturbing the network functionality excessively. The routing protocol keeps track of the whereabouts of the nodes in the network thus making it possible to route packets between distant nodes using the other nodes in the network as an intermediate nodes. Currently there exists several routing protocols that can be used in mobile mesh environment.
The routing protocols are able to handle small amounts of mobility. Still, problems arise when the majority of nodes are mobile, because every node has to know whereabouts of every other node in some extent. This becomes a more serious problem when mesh networks get larger because the changes in the network structure have to be informed to bigger amount of nodes and because the number of changes in the network is of course the higher the bigger the network is. Also the routing tables get bigger and bigger if the mesh gets larger because the routes in the network cannot be aggregated in the same way as in traditional wired networks.
FIG. 1 shows a wireless mesh network (WMN) comprising several wireless routers WR connected via radio links. Not all wireless routers WR hear each other. Hence, packets between two wireless routers WR in the same WMN might have to be forwarded via several wireless routers WR and wireless links before reaching the destination. A wireless router WR can also have other interfaces than those to the mesh network, e.g. WLAN, Ethernet and/or Bluetooth interfaces, to which other network devices are connected. Furthermore, there may also exist routers having other sub-networks with laptops LT, wired routers R and servers SV. One or more of the wireless routers WR may act as an AirHead AH arranged to connect the WMN to other larger networks, e.g. to the Internet. Terminals such as laptops LT may also include wireless router functionality, i.e. they may act as a part of the WMN.
WMNs suffer from constantly changing conditions, which cause links to fail and reborn according to the current conditions. They may not have any specific administrator to take care of the networks, which means that they have to be self-organizing and self-healing. In WMNs, the wireless routers WR can also move, which means that the network structure is changing all the time.
Currently, the existing mesh networks are quite static with mostly fixedly mounted wireless routers WR, but in the near future also cars, trains, buses and other public transport devices may join the WMNs. In the wildest visions, all the people walking on the streets have their own small wireless router WR in their pocket and the whole network consists of these privately owned devices. In this kind of situation, the network structure is changing really fast and there is no way of keeping track of the exact location of every device. Also the size of the WMN in both the number of wireless routers WR and the surface area may be quite huge. In extreme cases one WMN may cover the whole world. This means that the methods proposed to be used in future WMNs have to be as scalable as possible so that they can be used efficiently no matter how large the networks will be. The large address space of the new Internet Protocol version 6 (IPv6) makes it possible to use the IP protocol in this new environment. However, because the IP addresses of the routers cannot be changed on the fly easily without breaking the connections, this is not an optimal solution for the mobile wireless networks. The address changing on-the-fly would be needed in order to preserve the route aggregation in the IP networks.
New routing protocols have been developed especially for wireless networks with no fixed topology. These protocols try to decrease the amount of the routing traffic needed by not telling other nodes about the less important changes in the network structure. However the location of other nodes has to be known to some extent in order to be able to forward the traffic to the right direction. These protocols work quite fine when the network structure remains the same and only the radio quality of the links changes a little. But when the routers start to move, the usable network capacity collapses because of the routing protocol updates and the packets not finding their destinations.
The mobile IP protocol solves the terminal mobility problems that are caused by mobile terminals changing their location in the network. It changes the mobility problem into a routing problem hence not solving the mobility problems in the wireless mesh networks, where also the routers are able to move and where the main problems are actually in the routing area.
Furthermore, traditional MAC (Medium Access Control) switching in fixed wired networks is based on learning the MAC addresses from the source MAC addresses of the packets coming in from an interface. It also requires that broadcast and multicast packets have to be repeated to every link. This method works fine in the traditional fixed wired networks, but in WMNs it cannot be used, because of the different network structure and the network capacity wasted because of the broadcasts.